1. Viscoelastic properties of articular cartilage at high frequencies Geoffrey R Fulcher David WL Hukins Duncan ET Shepherd School of Mechanical Engineering University of Birmingham

2. Articular cartilage Low friction & wear Transmitting forces
Deformable: larger areas - lower stress

3. Articular cartilage Viscoelastic
Behaviour under the full range of physiological loading frequencies?

4. Rise time of heel strike force
Generally ms
Some people
5-25 ms
implicated in the onset of osteoarthritis

5. 1 Hz
rise time 500 ms
90 Hz
rise time 5.6 ms

6. Objective
To measure the viscoelastic properties of bovine articular cartilage at loading frequencies of up to 90 Hz

7. Viscoelasticity

8. Viscoelasticity phase angle of 0° material is purely elastic
material is purely viscous

9. Viscoelastic material
Storage modulus, E´
elastic part of the response (where energy is stored and used for elastic recoil of the specimen when a stress is removed)
Loss modulus, E´´
the viscous response (where energy is dissipated and the material flows)

10. Materials & methods
Bovine
Tibial plateau
Fluid bath
Indenter

11. Materials & methods Bose ElectroForce 3200 testing machine WinTest
DMA (Dynamic Mechanical Analysis)

12. Materials & methods
sinusoidally varying compressive force of between 16 N and 36 N
1.7 MPa
1 to 90 Hz
Calculated: E´
E´´ 

13. Results

14. Results

15. Storage modulus
Curve fit
Slope

16. Phase angle  > 0 for all frequencies 1 to 90 Hz
3.4 to 5.7° (mean = 4.9°, SD = 0.6°)
Viscoelastic

17. Implications for osteoarthritis?
More energy is stored by the tissue than is dissipated
Effect is greater at higher frequencies.
Main mechanism for this excess energy to be dissipated is by the formation of cracks

18. Implications for osteoarthritis?
1 Hz
100 Hz
10 Hz

19. Conclusions Articular cartilage is viscoelastic 1 to 90 Hz
High frequency loading, seen in some of the population, may be implicated in osteoarthritis

20. Acknowledgements Arthritis Research Campaign
Duncan ET Shepherd BEng, PhD, CEng, FIMechE
School of Mechanical Engineering
University of Birmingham